Fuel cells provide electricity from chemical oxidation-reduction reactions and possess significant advantages over other forms of power generation in terms of cleanliness and efficiency. Typically, fuel cells employ hydrogen as the fuel and oxygen as the oxidizing agent. The power generation is proportional to the consumption rate of the reactants.
A significant disadvantage which inhibits the wider use of fuel cells is the lack of a widespread hydrogen infrastructure. Hydrogen has a relatively low volumetric energy density and is more difficult to store and transport than the hydrocarbon fuels currently used in most power generation systems. One way to overcome this difficulty is the use of reformers to convert the hydrocarbons to a hydrogen rich gas stream which can be used as a feed for fuel cells.
Hydrocarbon-based fuels, such as natural gas, LPG, gasoline, and diesel, require conversion processes to be used as fuel sources for most fuel cells. Current art uses multi-step processes combining an initial conversion process with several clean-up processes. The initial process is most often steam reforming (SR), autothermal reforming (ATR), catalytic partial oxidation (CPOX), or non-catalytic partial oxidation (POX). The cleanup processes are usually comprised of a combination of desulfurization, high temperature water-gas shift, low temperature water-gas shift, selective CO oxidation, or selective CO methanation. Alternative processes include hydrogen selective membrane reactors and filters.
In a fuel processing unit, such as an SR or ATR reactor, superheated steam and preheated air and/or fuel are required. And the fuel processing efficiency is directly affected by the extent to which steam can be superheated and how hot air or fuel can be preheated.
Oftentimes, hot combustion exhaust (also called flue gas) from a combustor or an anode tailgas oxidizer (ATO) is used as the heating source for steam generation and for air/fuel preheating. In addition to the fuel reformer, a fuel processing system typically includes a boiler and an air/fuel preheater which consists of a shell with several spiral coils inside for heat exchanging and for steam generation. The hot combustion exhaust gas typically passes through the shell once, while water and cold gas (air or fuel) flow through the inside coils. In such a configuration, steam can be generated yet, the steam is not superheated hot enough to the desired temperature (for example 600° C.). Further, the steam production is not stable due to the easy formation of slug flow inside the coils. In addition, the heat transfer efficiency for gas preheating is very limited and as a result, air and/or fuel cannot be preheated hot enough for ATR applications.
The poor steam production and heat transfer efficiency may be attributed to facets of the design of the boiler and air/fuel preheater. First, the combusted exhaust gas passes through the shell only once which does not provide for sufficient contact time between the hot exhaust gas and the cold streams. Second, the typical boiler utilizes a ¼ inch spiral coil, and the moving of two-phase flow inside the coil is very close to horizontal, as a result, slug flow is more likely formed in so small diameter coils. Third, most of the time, the hot and cold streams flow co-currently, which limits the cold streams from being heated to a temperature above the exit temperature of the ATO exhaust.
A design for a boiler and a gas preheater which has both effective heat transfer and stable steam production is needed. The present invention provides a heat transfer unit for steam generation and gas preheating. Optionally, the heat transfer unit of the present invention may be located downstream of a combustor or an ATO in fuel processing applications.